Air Pollution Control Systems
All combustion plants emit a certain quantity of air pollutants. Smoke, carbon monoxide, odour and hydrocarbons are all products of poor combustion and can be reduced to a negligible level by the use of a properly designed and operated secondary chamber. Other pollutants require the use of additional exhaust gas cleaning equipment.
The types and concentrations of contaminants in the flue gases of hospital and other waste incinerators depend on incinerator design, the waste being burned and combustion conditions. Flue gas contaminants can generally be categorised as follows:
- Particulate matter
- Acid gases
- Heavy metals
- Products of incomplete combustion
Particulate Matter
Particulate matter consists primarily of entrained non-combustible matter in the flue gas, as well as some of the products of incomplete combustion that exist in solid or aerosol form.
The quantity of particulates discharged from the incinerator is dependent on the ash content of the waste, the gas velocities through the incinerator and the type of ash agitation used.
Acid Gases
The quantities of acid gases discharged from the incinerator are dependent on the type of waste burnt. The main sources of acid gases when incinerating biomedical wastes is from PVC which contains approximately 54% w/w chlorine. Any chlorine present will react with hydrogen or water to form hydrochloric acid. This can only effectively be removed by absorption and neutralization with alkali scrubbing.
Heavy Metals
Metals, such as Pb, Cd, As, Ni, Zn and Hg, are present in the flue gas primarily as oxides and chloride salts. Most of these compounds are in the vapour phase within the incineration system, since these compounds boil or sublime at temperatures around 1,000°C. The metal compounds tend to condense as the flue gas is cooled and they become absorbed on fine particulate matter. The more volatile metals such as mercury and lead may remain in the vapour phase, depending upon cooling temperature.
Products of Incomplete Combustion
Products of incomplete combustion include carbon monoxide and trace organics. PIC`s are highly dependent on constituents in the waste feed and the combustion conditions. Depending on temperature some of the constituents may condense on fine particulates during cooling. A group of organic compounds that is of interest because of their toxicity are PCDD`s and PCDF`s. The main mechanism for the production of these compounds is the catalytic reaction of chlorine and organic compounds on the surface of particulates in the temperature range 400–200°C. By ensuring virtually complete combustion of wastes and by providing a rapid transition through this temperature range the emission of these pollutants can be minimised to negligible levels.